• Electricity can be produced when water is passed over a nanolayer of iron that is oxidized in air to form a metal oxide nanooverlayer at a variable rate.
• Higher levels of current were achieved when the concentration of sodium chloride in water and the flow rate of water over the metal oxide nanooverlayer are varied. 
• Iron, nickel and vanadium demonstrate much better results than other metals because their oxides are present in multiple oxidation states. 

The wide availability of water is leading researchers to look for ways to utilize ions flowing in the aqueous environment to generate electricity when placed in contact with a specific surface. The objective is to develop a cost-effective approach for generating electricity that is environmentally friendly.

In a previous TLT article, researchers found that an electric voltage could be produced when deionized water flowed over a superhydrophobic surface (1). The researchers increased the voltage through application of a fluorinated lubricant and addition of sodium chloride at a dilute concentration in deionized water. Electricity was generated in the 50-100 millivolt range.

Franz Geiger, Dow professor of chemistry in the chemistry department at Northwestern University in Evanston, Ill., says, “Past efforts to generate electricity involved passing water droplets over graphene—a semi-conductor—and other dielectric-semiconductor structures. Efficiencies in the range of 30% have been reported, which places this technology at a comparable level to solar cells.”

The problem with using graphene is the cost incurred through scaling up the process out of the lab. Geiger says, “Multistep fabrication is needed to prepare nanolayers of graphene that are suitable for use in this application, which will increase the cost. A second concern is that moving water at a high rate of speed over a graphene surface can cause oxidation and delamination, leading to potential performance issues.”

Geiger believes that graphene is too finicky for use as a surface to generate electricity. Thomas Miller, professor of chemistry at the California Institute of Technology in Pasadena, Calif., outlined the three pillars that must be met. He says, “The surface must be low in cost, readily scalable to a commercial level and robust.”

To meet these criteria, Geiger and Miller have turned to a readily available metal, iron.

Metal nanolayers
The researchers determined that application of a nanolayer of iron onto a solid or flexible substrate (such as glass, plastics or polymers) followed by passivation in air to form a metal oxide nanooverlayer is an excellent candidate when water is passed across it at a variable rate. Geiger says, “We have been working with iron oxide nanofilms and found that a voltage was detected when a drop of water was applied to the surface.”

An important aspect was the need to initially produce a pure metal nanolayer. Geiger says, “We prepared metal nanolayers by applying iron to a substrate through the use of physical vapor deposition. The nanolayers produced were 5 to 20 nanometers thick. Iron layers that contain small percentages of magnesium, calcium and zinc atoms cannot be used because these impurities will react with water to form hydroxyl groups that will lead to rapid corrosion, interfering with the power generation process.”

Once the metal nanolayer is prepared, a self-oxidation process takes place in a similar manner to a metal surface rusting leading to the spontaneous formation of an oxide coating that is a few nanometers thick. The researchers initially applied the iron nanolayers on a 2.5 centimeter by 7.5 square centimeter slide, which was placed in a small polytetafluoroethylene cell that contained a flow channel (6 millimeter by 7.5 millimeter by 35 millimeter) so that water with a varying concentration of sodium chloride can be in contact with the metal nanolayer.

An experimental setup is shown in Figure 3. 


Figure 3. The experimental setup shown was used to demonstrate that water passing over a metal oxide nanooverlayer at a variable rate can produce electricity. (Figure courtesy of Northwestern University.)

The researchers varied the concentration of sodium chloride used in water that passed over the metal nanolayer from 0.1 millimolar to 2.0 molar. The flow rate of the sodium chloride containing water was 20 milliliters per minute. This led to the generation of current in the 0.2 microampere range and voltages in the millivolt range. 

Varying the sodium chloride concentration and changing the flow rate both lead to the production of higher levels of current. Besides iron, the researchers evaluated other metals including nickel and vanadium and found similar results. But, when using aluminum and chromium, a much lower current density was found. 

Miller says, “We believe the reason that iron, nickel and vanadium show much better results is because their oxides are present in multiple oxidation states while aluminum and chromium oxides only have one oxidation state. We believe that this mechanism involves two processes. Charge fluctuations in the metal oxide nanolayer lead to a surface capacitance that causes current to be generated when the sodium chloride solution flows over the oxide. And this effect is potentially enhanced by electron transfer processes within the nanolayer, which is facilitated by oxides with multiple oxidation states.”

Geiger says, “The reason this approach works well is because the metal nanolayer/metal oxide nanooverlayer is a nice, clean system that can be prepared very easily, in a single step, -from a single element, and at a low cost. This system is chemically pure and very robust.”

The researchers will be evaluating the efficiency of the process in the future. Geiger says, “As we look to commercialize this process, we must figure out how to minimize the possibility of biofouling. One possibility is to add biocidal metals such as silver.

This system is well suited for use in extreme environments. Geiger points out that one potential application is to place a coating on an implantable stent that can utilize the flow of blood to power a device. 

Additional information can be found in a recent article (2) or by contacting Geiger at f-geiger@northwestern.edu and Miller at tfm@caltech.edu. 

REFERENCES
1. Canter, N. (2019), “Electricity generated from salt water and superhydrophobic surface,” TLT, 75 (2), pp. 12-13.
2. Boamah, M., Lozier, E., Kim, J., Ohno, P., Walker, C., Miller, T. and Geiger, F. (2019), “Energy conversion via metal nanolayers,” Proceedings of the National Academy of Sciences, 116 (33), pp. 16210-16215. Pre-edited final version available free of charge at arXiv:1907.13170.